This invention concerns thermoforming machines which use vacuum and/or air pressure to cause preheated sheet plastic material to be forced into conformity with mold cavities in order to form such products as plastic cups. A number of such products may be formed simultaneously from plastic sheet material inserted across an array of mold cavities. The formed products are cut free from the sheet by trim tooling. Thermoforming machines feed the sheet material into the forming station either in precut sheets, or in a continuous sheet feed off a roll, typically where thinner material is being used.
In trim-in-place roll fed machines the trimming is performed in the same station whereat the products are molded. The combined mold and trimming tools are very costly.
In post trim roll fed machines, where larger machines are the norm, standard practice has been to transfer the sheet material out of the mold station and into a separate trim press where the products are die cut from the sheet. This has the advantage of reducing the precision tooling tolerances otherwise required in the thermoforming tooling to thereby reduce the cost of the tooling.
Plug assists are used for products having deep cavities, comprising an array of plug shaped members, each mounted opposite a respective mold cavity, which are advanced into the sheet material during thermoforming, aiding in the movement of the sheet material into the mold cavities, as this prevents undue localized thinning of the material as it is stretched into conformity with the mold cavity under the influence of vacuum and/or a differential air pressure.
The forming station is located in a press having upper and lower platens, one platen carrying the mold tooling, the other carrying tooling defining sealing edges engaging the plastic sheet to create a sealed cavity around each sheet section to be formed, enabling the air pressure to stretch the material in the mold cavity and thereby form the product. Clamping of the sheet material section is also carried out as a part of the process for controlling material stretch during the thermoforming process in conjunction with the effect of the plug assists.
In post trim roll fed machines, the plug assists were in the past advanced by the press platen motion. In machines in which trimming was carried out in the forming machine, the plug assists of necessity were operated by a so-called xe2x80x9cthird platenxe2x80x9d, i.e., by a separate motion from the main platen due to the need to carry out die cutting by the main platen motion.
Recently, it has been found that to carry out plug assist motion by a third platen is advantageous, as the main platen motion can be optimized for rapid travel and powerful sheet clamping, freed from the constraints imposed by the need to carry out the plug assist motion, which is instead carried out independently by the motion of the third platen.
In the application of the third platen drive to the plug assists for roll fed machines, the spider plate has been mounted on the main platen, enclosed within a sealed box containing the spider plate and plug assists. The box must be sealed since the presence of the spider plate prevents individual sealed chambers from being provided around each plug assist.
Clamping rings are also required, capable of applying clamping pressure to sections of the sheet material around each mold cavity. In this design, the clamping rings for each cavity are attached to the top of the sealed box which is unsupported internally. The box interior is pressurized in order to pressurize the space within each clamping ring to exert air pressure on the sheet material to carry out the thermoforming process. The air pressure is exerted on the sheet material through holes in the top of box. The holes also accommodate the plug assists when these are stroked up into contact with the sheet material.
This arrangement has certain disadvantages, including the need to pressurize the entire volume of the interior of the box with each machine cycle, as the pressure is vented when the platens are retracted, and this consumption of compressed air represents a significant power usage.
Another disadvantage is that the use of the top plate of the box as a support for the clamping rings sometimes leads to cracking of the rings since the top of the box is unsupported across much of its span, and thus may not be rigid enough to avoid overstressing of the clamping rings.
Yet another substantial drawback is that the spider plate connecting all of the shafts to which the plug assists are attached and which the third platen drives, is completely enclosed within the sealed box so as to not be able to be observed. This is important since the third platen stroke must be adjusted, and a collision with the mold tooling or the base plate has in the past occurred due to overstroking of the third platen in either direction.
While other third platen arrangements have been used in trim-in-place thermoforming machines which would avoid these disadvantages, the relatively low shut heights of the prior roll fed post trim machines would preclude the other arrangement since an increased shut height is required.
It is the object of the present invention to provide an arrangement for third platen stroking of the plug assists for post trim thermoforming machines which avoids the above described disadvantages of the third platen plug assist actuation heretofore used on such machines.
The above object and others which will become apparent upon a reading of the following specification and claims are achieved by a tooling assembly having an array of cylindrical chambers, each receiving a plug assist, with an attached rod for each plug assist extending down through a clamp ring support plate assembly.
Clamping rings are defined around the top of each chamber for clamping sections of the sheet material around the mold cavity located above each chamber.
The plug assist rods extend through openings in the support plate and are sealed to the openings. The plug assist rods are connected together below the support plate by a spider plate, in turn having a series of shafts constituting a driving connection to the third platen.
The press shut height is increased to accommodate the increased overhead clearance required for this spider plate arrangement, in prior post trim machines.
A series of standoffs extend between the support plate and the main platen so as to force the clamping rings into engagement with sheet material sections by main platen stroking during each machine cycle.
Only the clearance between the plug assists and surrounding chambers is pressurized to drastically reduce the consumption of compressed air.
The standoffs provide a rigid support for the clamping plate to prevent cracking of the clamping rings. The spider plate is exposed to be viewable during adjustment of the plug assist stroking in either direction.